Contactless Readers operating in the ISM frequency band at 13.56 MHz carrier frequency offer a contactless interface. This interface offers power for transponder chip operation and a half-duplex bi-directional communication to contactless Transponder cards (e.g. Smartcards, e-Passports, Ticketing, etc.) or Near Field Communication (NFC) devices. To guarantee interoperability between contactless Reader and Card/Transponder devices of different manufacturers, international standards specify the properties of the air interface, e.g. ISO/IEC14443 (the proximity base standard)+ISO/IEC10373-6 (the test standard for proximity systems), or EMVCo (the industry standard for payment, also for contactless payment), or ECMA 340, or NFC Forum. Conformance of products to these standards is essential, and especially for products in governmental use (e.g. e-Passport) this has to be certified by an accredited test Lab. Several properties are specified for the air interface of such contactless products and tested values have to be in a range defined by the standard.
In inductive coupled systems a reader generates a magnetic field that typically transfers energy, clock, and data to the transponder. The minimal field strength needed to supply and communicate with the transponder may typically decay with approximately 60 dB/dec in the near field.
Card-Mode: For card devices without power supply, the communication distance is typically limited by the minimal field strength to supply the card. For card devices with power supply (e.g. NFC devices in target-mode), the communication distance is typically limited by the minimal receiver voltage that allows a correct decoding of the reader command. In inductive coupled devices the voltage at the card receiver depends on the distance to the reader, the coupling factor, and the generated field strength of the reader device. The cards are typically designed to operate within a certain field strength range. Too large field strengths may destroy the device and too small field strengths are insufficient for a successful communication. A stabilization of the receiver input voltage may increase the dynamic range with respect to the operational field strength range. Thereby, for a given maximal field strength the communication distance can be increased or for a given communication distance the maximal field strength can be increased.
Reader-Mode: The antenna matching circuit of a reader device is detuned due to cards within the generated magnetic field. As a consequence the amplitude of the card response on the reader receiver input is reduced, which degrades the reception quality and thus the communication distance. Also in this case, a stabilization of the receiver input voltage to a constant value may result in a maximal receiver input signal, which is independent from de-tuning of the reader antenna.
US 2006/0186995 A1 describes transceivers for RFID readers and RFID transponders with an automatic gain control (AGC) circuit that adjusts the receiver gain according to the signal strength.
In all of the above cases, it is on the one hand desirable to use a voltage regulation or AGC system that provides a rapid correction of the receiver input voltage. On the other hand, however, using a rapidly adapting control system may influence the modulated data content of the received signal and thereby cause bit errors or even complete receiving failure.
There may thus be a need for a way of effectively stabilizing the receiver input voltage while at the same time avoiding negative influences on the modulated data content of the received signal.